The developers over at XDA have been hunting through a system dump of Android 4.1 Jelly Bean, digging out some of the updated Google apps, and making them available for users of rooted ICS devices.
First came Google Sound Search (also known as Google Ears) which identifies the song playing (like Shazaam / Soundhound) and links to the album in the Google Play Store.
Now the new Google Play Music app is available, with an updated design and greatly improved widget.
Be warned that I had to uninstall the previous music app before it would let me install the new version. Then it didn’t seem to be connecting to my online music, with no useful options available in the settings. So I’d recommend taking a backup of your current Google Music app before trying to install this version.